1. Field of the Invention
This invention relates to methods and apparatus for depositing transparent, electrically-conductive metal oxide coatings on non-conducting substrates, such as glass, by reactive sputtering, and to articles having such coatings deposited on them. By way of example, the article may be a windscreen, e.g. an aircraft windscreen, on which the coating can provide electrical resistance heating means for de-misting or de-icing.
2. Description of the Prior Art
The method of reactive sputtering of a metal oxide coating on to a substrate is known, in which a substrate is maintained at a controlled elevated temperature in a vacuum vessel containing an atmosphere of oxygen and another gas or gases at a controlled reduced pressure, and a high negative potential is applied to a cathode assembly of the metal which is arranged in the vicinity of the substrate and presents a surface or surfaces extending substantially parallel to the substrate surface which is to be coated with the film.
When depositing such films on substrates of extended lateral dimensions (e.g. 30 cm. and upwards), in order to avoid undesirable variations in the characteristics of the film, it has been found necessary to take steps to provide access for the sputtering atmosphere to penetrate into the whole of the working space between the cathode assembly and the substrate, so as to maintain a substantial degree of uniformity in the oxygen concentration in the working space. One method of providing such access, as described and claimed in co-pending U.S. Patent Application Ser. No. 220,899, has been to divide the cathode assembly into spaced parallel strips so as to provide passages for the sputtering atmosphere between the strips, and to provide relative reciprocating movement between the strips and the substrate in a direction transverse to the length of the strips and parallel to the substrate surface, of an amplitude substantially smaller than the overall length of the cathode assembly but sufficient to cause all parts of the substrate surface to be coated by sputtering from at least one of said parallel strips for equal deposition periods during the deposition process. For example, the amplitude may be equal to the spacing between the centre lines of adjacent strips. For a substrate which is curved along its length, the strips may be reciprocated on similarly curved tracks. In this way it has been possible to deposit films of substantially uniform thickness and specific resistivity on relatively large substrates.